Preparation and microwave characterization of submicrometer-sized hollow nickel spheres

Article abstract of DOI:10.1016/j.jmmm.2005.11.006

The hollow structural submicrometer-sized nickel spheres were successfully fabricated by the autocatalytic reduction method. Because of the metallic and ferromagnetic behaviour of the nickel spheres, the low-density microspheres could obtain high dielectric constant and magnetic loss in microwave frequencies. The abrupt variation of the real part and the sharp peaks of the imaginary part of permittivity and permeability were observed for the micrometer-sized and nanometer-sized nickel hollow spheres. Reflection loss less than -25 dB were predicted over 11 GHz with a thickness of 1.5-2.0 mm.

Full text of DOI:10.1016/j.jmmm.2005.11.006

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Journal of Magnetism and Magnetic Materials 303 (2006) 181–184
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Preparation and microwave characterization of submicrometer-sized
hollow nickel spheres
Ã
Yida Deng, Xi Liu, Bin Shen, Lei Liu, Wenbin Hu
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, PR China
Received 8 August 2005; received in revised form 24 October 2005
Available online 28 November 2005
Abstract
The hollow structural submicrometer-sized nickel spheres were successfully fabricated by the autocatalytic reduction method. Because
of the metallic and ferromagnetic behaviour of the nickel spheres, the low-density microspheres could obtain high dielectric constant and
magnetic loss in microwave frequencies. The abrupt variation of the real part and the sharp peaks of the imaginary part of permittivity
and permeability were observed for the micrometer-sized and nanometer-sized nickel hollow spheres. Reflection loss less than ꢀ25 dB
were predicted over 11 GHz with a thickness of 1.5–2.0 mm.
r 2005 Elsevier B.V. All rights reserved.
Keywords: Submicrometer-sized particles; Hollow spheres; Autocatalytic reduction; Microwave absorption
1. Introduction
absorbers were very limited. In the present study, the
microwave absorption behaviours of the submicrometer-
sized hollow nickel particles by an autocatalytic reduction
method are reported.
There is intensive interest in the preparation of electro-
magnetic (EM) materials with low reflectivity recently.
Because EM interference has been a serious problem due to
explosive growth in the utilization of electronic devices in
industrial, commercial, and military applications [1]. One
way to make EM absorbers is to disperse magnetic
powders including ferrites and iron, cobalt, and nickel
metal or alloy powders in an insulating matrix [2,3].
Previous studies [4,5] showed that the microwave proper-
ties of EM materials largely depend on particle shape,
granulometry, and structure. And increasing interest has
been given to the synthesis of micrometer- and nanometer-
sized EM materials with specific morphologies in recent
years.
There were also reports [6–10] on submicrometer-sized
particles with hollow structure often exhibiting novel
properties dramatically different from their solid counter-
parts in the fields of chemistry, physics, biotechnology, and
materials science, and thus making them attractive from
both scientific and technological viewpoints. However,
studies of hollow particles for the application in microwave
2. Experimental methods
2.1. Preparation of hollow nickel spheres
By means of an autocatalytic reduction method [11],
hollow nickel spheres were prepared with analytically
pure NiSO4 ꢁ 6H2O, NaH2PO2 ꢁ H2O, NaOH, acetic acid
and citric acid as starting materials. During the preparation
process, 20 g nickel sulfate was dissolved in 200 ml solution
containing 0.1 g acetic acid and 0.15 g citric acid to prepare
a mixed nickel solution. About 25 g sodium hydroxide
and 3 g sodium hypophosphite were dissolved in 200 ml
de-ionized water in beakers, respectively. And the solutions
were preheated at 80 1C for 5 min. Then the nickel solution
and alkali solution were mixed by violent stirring, and a
viridescent colloid was produced. And then the sodium
hypophosphite solution was added to the as-prepared
colloid by evenly stirring. Finally, a dark-gray powder
was obtained after the reaction had taken place for
2–5 min, and the powder was repeatedly washed with
Ã
Corresponding author. Tel.: +86 21 6293 3585; fax: +86 21 6282 2012.
E-mail address: wbhu@263.net (W. Hu).
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2005.11.006

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Y. Deng et al. / Journal of Magnetism and Magnetic Materials 303 (2006) 181–184
182
ammonia and de-ionized water. The final products were
dried in vacuum furnace at 100 1C for 2 h.
3. Results and discussion
In our studies, the submicrometer-sized hollow nickel
spheres were fabricated by the autocatalytic reduction
method [11]. The redox reaction used in electroless nickel
(EN) deposition process was catalyzed around the nickel
hydroxide (Ni(OH)2) colloidal particles and the metal
nickel assembled around the particle surfaces and the
colloidal particles sacrificed during the process. Typical
examples of nickel particles obtained by autocatalytic
reduction method are provided by the FESEM micro-
graphy in Fig. 1. The particles are perfect spherical with
the mean diameter dm ¼ 1 mm (Fig. 1(a)) and dm ¼ 60 nm
(Fig. 1(b)), and show the narrow size distribution and the
low degree of agglomeration. The colloid nuclear give
particles in the submicrometer-size range and allow their
mean diameter to be controlled by varying the concentra-
tion of NaOH used to form the colloid particles [11].
A broken sphere (Fig. 1(a) inset) was obtained by slightly
grinding the samples shown in Fig. 1(a), demonstrating
the samples with the hollow inner and the shell outside.
A hollow nanometer-sized nickel spheres was shown in
Fig. 1(b) inset by the TEM.
2.2. Characterization techniques
Field-emission scanning electron microscopy (FESEM)
measurements were performed with a Philips XL30FEG
operated at an accelerating voltage of 10 kV. FESEM
samples (on silicon surface) were coated with about 5 nm
Au. Transmission electron microscopy (TEM) images were
recorded on a Philips CM200FEG microscope operating at
50 kV. Samples for TEM were sonicated in acetone
for 10 min (to redisperse the hollow spheres) and subse-
quently deposited onto a carbon grid. Structural char-
acterization was performed by means of X-ray diffraction
using a D/Max-gB diffractometer with Cu-Ka radiation at
an accelerating voltage of 40 kV.
The composite samples were prepared by homoge-
neously mixing the nickel powders with polypropylene in
a volume ratio (volume of nickel powder/volume of
polypropylene) of 1:1. The mixture was made into
toroidal-shaped samples with an outer diameter of
7.0 mm and inner diameter of 3.0 mm. The complex
permeability and permittivity of composite were measured
using an Agilent E8362B PNA network analyzer in the
frequency range of 2–18 GHz.
From X-ray diffraction (XRD) patterns (Fig. 2) it can be
inferred that both the diffraction peaks of the micrometer-
sized and nanometer-sized spheres were broadened and the
nanometer-sized spheres show a broader peak. The XRD
Fig. 1. FESEM micrographs of micrometer-sized (a) and nanometer-sized (b) nickel spheres were obtained by autocatalytic reduction. A broken sphere
can be seen in (a) inset, and a hollow structural nanometer-sized sphere by TEM is shown in (b) inset.
(111)
(220)
(200)
micrometer-sized nickel spheres
nanometer-sized nickel spheres
20
40
60
80
°
2θ/
Fig. 2. XRD pattern indicated the Ni spheres were the face-centered cubic (FCC) structure.

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Y. Deng et al. / Journal of Magnetism and Magnetic Materials 303 (2006) 181–184
183
patterns of the samples indexed both the micrometer-size
and nanometer-size nickel spheres as the face-centered
cubic (FCC) lattice. It is well known that nickel crystallize
with a FCC lattice and a ¼ 0:3528 nm [12]. However, the
as-synthesized spheres show their lattice parameters are
0.35147 nm (micrometer-sized particles) and 0.35102 nm
(nanometer-sized particles). These phenomena indicate
that the spheres are composed of nanocrystallites and the
crystallites have distinct shrinkage.
Fig. 3 shows the real and imaginary of permittivity e0r,er00
of nickel hollow spheres–polypropylene composites. It is
interesting that the curves of the real part of permeability
exhibit the abrupt decrease, while the imaginary part of
permeability shows the sharp peak at the corresponding
frequency. This suggests a resonance behavior, which is
expected when the composite is highly conductive and
skin effect becomes significant [13]. In our studies, the
resonant frequency of the nickel spheres composite is
related to the hollow structure and the high conductivity
of nickel. However, the exact mechanism yet is not
ascertained. The decreasing amplitude of e0r is higher
for the micrometer-sized nickel spheres composites than
the nanometer-sized ones, and the abrupt decrease shifts
to higher frequency. The higher value of dielectric con-
stant is attributed to better metallic behavior of micro-
meter-sized nickel spheres compared with nanometer
spheres.
The real and imaginary part of permeability m0r,mr00 of
nickel hollow spheres–polypropylene composites is shown
in Fig. 4. As shown in the figures, the real part and
imaginary part of the complex permeability also exhibit a
sharp curve at the resonance frequency regions. The value
of m0r decreases at frequency p9 GHz for composites of
micrometer-sized nickel hollow spheres and at frequency
p12 GHz for composites of nanometer spheres, and the
abrupt increase occurs subsequently. The sharp peaks of mr00
are observed at frequency 10.6 GHz for composites of
micrometer-sized nickel hollow spheres and at 13.8 GHz
for composites of nanometer spheres. Such behavior may
be related to the resonance frequency of the nickel hollow
spheres, and the shift of the resonance frequency can be
interpreted by a variation of particle size [14].
The normalized input impedance Zin of a single metal-
backed microwave absorbing layer is given by [15]
rffiffiffiffi
pffiffiffiffiffiffiffiffi
2pfd mrer
mr
er
Zin
¼
tanh
,
(1)
c
where mr and er are the relative complex permeability and
permittivity, respectively, of the composite medium, c the
velocity of electromagnetic waves in free space, f the
35
40
micrometer-sized nickel spheres
nanometer-sized nickel spheres
micrometer-sized nickel spheres
nanometer-sized nickel spheres
30
25
20
15
10
5
35
30
25
20
15
10
5
0
2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0G
Frequency (Hz)
2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0
Frequency (Hz)
Fig. 3. Frequency dependence of the real part and imaginary part of the complex permittivity of micrometer-sized and nanometer-sized nickel hollow
sphere composites.
2.0
micrometer-sized nickel spheres
nanometer-sized nickel spheres
micrometer-sized nickel spheres
nanometer-sized nickel spheres
1.6
1.2
0.8
0.4
0.0
2.0
1.5
1.0
0.5
2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0G
2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0G
Frequency (Hz)
Frequency (Hz)
Fig. 4. Frequency dependence of the real part and imaginary part of the complex permeability of micrometer-sized and nanometer-sized nickel hollow
spheres composites.

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Y. Deng et al. / Journal of Magnetism and Magnetic Materials 303 (2006) 181–184
184
5
0
5
0
-5
-5
-10
-15
-20
-25
-10
-15
-20
-25
-30
t = 1.0mm
t = 1.5mm
t = 2.0mm
t = 1.0mm
t = 1.5mm
t = 2.0mm
-30
-35
0.0 2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0G 20.0G
Frequency(Hz)
0.0 2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G 16.0G 18.0G 20.0G
Frequency(Hz)
(a)
(b)
Fig. 5. Frequency dependence of the reflection loss of the nickel hollow spheres: (a) micrometer-sized and (b) nanometer-sized composites at various
sample thickness.
frequency of microwaves, and d the thickness of the
absorber. The reflection loss is related to Zin by [15]
nickel hollow spheres. Reflection loss less than ꢀ25 dB
were predicted over 11 GHz with thickness of
a
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1.5–2.0 mm. This absorber is well advanced in both mass
and thickness in comparison with the conventional ferrite
absorber.
Zin ꢀ 1
Zin þ 1
RL ¼ 20lg
.
(2)
Thus, the surface reflectance of an absorber is a function
of six characteristic parameters, viz., m0r,mr00,er0 ,er00, f, and d.
Fig. 5 shows the calculated reflection loss as a function of
frequency for micrometer-sized and nanometer-sized nickel
hollow spheres composites of different thicknesses. The
calculations use the actual values of er and mr as shown in
Figs. 3 and 4. The reflection loss is found to depend
sensitively on the thickness of the absorber. The maximum
attenuation of the incident wave is predicted for a thickness
of 1.5 mm for micrometer-sized nickel hollow spheres
composites, and 2.0 mm for nanometer spheres. The
minimal reflection of the nanometer hollow spheres
composites is decreased with an increase in the thickness
of the absorber (d ¼ 2:0 mm, RL ¼ ꢀ34:5 dB); however,
the micrometer spheres composites have the minimal
reflection loss (ꢀ30 dB) in the thickness at 1.5 mm. It is
suggested that the reflection loss is related to a matching
thickness.
Acknowledgments
This work is supported by National Natural Science
Foundation of China (Grant no. 50474004) and Shanghai
Science and Technology committee Nano Special Fund
(Grant no. 0452nm046).
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